Thermoplastic resins such as polyester resins, polycarbonate resins and polyamide resins are widely used as various containers, films, electric/electronic parts, etc. owing to their excellent mechanical properties, heat resistance, moldability and recyclability. Among them, polybutylene terephthalate, polypropylene terephthalate and polyethylene terephthalate as polyester resins are widely used as materials to be molded into industrial articles such as connectors, relays and switches of automobiles and electric/electronic apparatuses, since they are also high in the reinforcing effect by an inorganic filler and excellent in chemicals resistance.
However, in recent years, there are growing demands for smaller and lighter-weight industrial moldings, and especially polybutylene terephthalate used for automobiles and electric/electronic apparatuses is requested to be enhanced in the flowability during melting without lowering its mechanical properties in view of these demands.
JP 2001-200038 A (claims) describes a polyester resin with a melt tension of 0.8 to 5.0 g containing a trivalent or higher polyvalent carboxylic acid or a polyhydric alcohol, but the obtained polyester resin has a problem that the viscosity increases to lower the flowability.
Further, JP 7-304970 A (claims) describes a method for enhancing the flowability by melt-mixing a combination consisting of a specific thermoplastic resin and a specific compound having at least three functional groups, but the flowability enhancing effect is insufficient while the mechanical properties also tend to decline.
Furthermore, JP 10-60240 A (claims) describes that a polyester resin composition consisting of polybutylene terephthalate resin, styrene-based resin and inorganic filler is excellent in low warping property and mechanical properties. However, this polyester resin composition has a problem that since the flowability is low, it cannot respond to the demands for smaller and lighter-weight industrial moldings in recent years.
On the other hand, in the conventional situation where more complicated shapes are being employed for products, respective parts are bonded to each other by an adhesive or mechanically bonded by bolts, etc. However, the use of an adhesive has a problem concerned with adhesive strength, and the mechanical bonding by use of bolts, etc. have such problems as cost, inefficient tightening work and weight increase. Hot plate welding has a problem of stringing, and vibration welding and ultrasonic welding require the disposal of the burrs formed near the joints. On the other hand, in the laser welding of molded resin parts, the overlapped resin parts are irradiated with a laser beam in such a manner that the laser beam is transmitted through one resin part, while the other resin part absorbs the laser beam, to be molten for fusion bonding. This technique has such advantages as three-dimensional bonding possibility, contactless processing and no burring, and is being rapidly employed in wider fields owing to these advantages.
Meanwhile, polyester resins, especially polybutylene terephthalate-based resins are very low in laser beam transmittance compared with thermoplastic resins such as nylon resins, and in the case where the laser welding technique is applied with a polybutylene terephthalate-based resin as the molded resin part through which the laser beam is transmitted, the thickness control is severe since the laser beam transmittance is low. Therefore, the molded resin part must be thinned for enhancing the laser beam transmittance, and the degree of freedom in product design is very low.
JP 2003-292752 A (claims) describes a case in which polybutylene terephthalate resin or a polybutylene terephthalate-based resin consisting of polybutylene terephthalate and a polybutylene terephthalate copolymer and one or more resins selected from a polycarbonate resin, acrylonitrile/styrene copolymer, polyphenylene oxide, styrene resin, acrylic resin, polyethersulfone, polyarylate and polyethylene terephthalate are mixed to enhance the laser beam transmittance, for allowing laser welding. WO 2003/085046 (claims) describes a case in which a resin composition consisting of polybutylene terephthalate resin and one or more selected from a polycarbonate-based resin, styrene-based resin and polyethylene terephthalate-based resin can have a higher laser transmittance and can be used for laser welding. In these cases, the laser beam transmittance can be greatly enhanced compared with that of polybutylene terephthalate resin alone, but there are such problems that the flowability is not always sufficiently high and that when the size of the molded article is large, molding cannot be performed.
It could therefore be helpful to provide a thermoplastic resin composition and a molded article thereof, respectively excellent in flowability and mechanical properties, and, in a preferred mode, also excellent in low warping property, laser transmittance and laser weldability.